Metals have many favorable properties, a primary advantage being their strength and resilience compared to other materials. However, metals are not as easily formed as many other materials, and in particular, while it is relatively straightforward to form sheets, plates, rods, and other symmetrical shapes, it is difficult to form shapes that are more complex. Thus, it is often necessary to join one or more pieces of stock metal to form a finalized product of an irregular shape. Of the available methods, welding provides the best compromise of efficiency and strength for joining most metals, especially steel. However, the welding process typically leaves unwanted stresses within the final part, increasing the risk of a later failure of the part. For critical load-bearing pieces such as machine frames, beams, levers, and arms, the risk of failure should be minimized, both to avoid costly repair as well as to safeguard operators, nearby personnel and critical processes.
In the past, it was known to heat treat metal to eliminate stresses, with the hopes that the stress-free metal would more easily resist cracking and breaking after a period of use. For example, U.S. Pat. No. 4,418,258 entitled “Method for Heat Treating Metal,” which recognized a need to relieve stress in microstructure welds, employed a heat treatment procedure for low alloy steel by induction heating in a second post-weld heat treatment. In particular, electrical induction heating, which is monitored, e.g., using a radiation pyrometer, is briefly applied to the area to be stress relieved in a post-weld heat treatment at a temperature approaching the critical point of the material. However, while this technique may reduce residual tensile stresses, it does not utilize the geometry or phase change of the material to induce beneficial stresses.
This background section is presented as a convenience to the reader who may not be of skill in this art. However, it will be appreciated that this section is too brief to attempt to accurately and completely survey the prior art. The preceding background description is a simplified narrative and is not intended to replace the reference being discussed. Therefore, interested readers should refer directly to U.S. Pat. No. 4,418,258 instead of relying upon the foregoing simplified narrative. Moreover, the resolution of deficiencies, noted or otherwise, of the prior art is not a critical or essential limitation of the disclosed principles.